iv. Optical Fiber

Optical Fiber plays a complete different set of rules than other bounded media. You won’t find any electricity here. Instead, fiber-optic cabling uses pulses of light (photons) for network communications, i.e. it relies on photonics instead of electronics. As a result, it is completely immune to EMI (Electro Magnetic Interference) and is extremely fast. The benefit of communicating with optic fibers is that they offer a drastic increase in data capacity. This increase in data capacity is due to several factors: First, optic fibers are physically much smaller than competing technologies. Second, they do not suffer from crosstalk which means several hundred of them can be easily bundled together in a single cable. Lastly, improvements in multiplexing have led to an exponential growth in the data capacity of a single fiber. Assisting communication across many modern optic fiber networks is a protocol known as Asynchronous Transfer Mode.
Fiber Optical cabling pertains the transmission of light through hair-thin, transparent fibers. As mentioned earlier, data travel in the form of light. Light signals that enter at one end of a fiber travel through the fiber with very low loss of light, even if the fiber are curved. A basic fiber-optic system consists of a transmitting device (which generates the light signal), an optical-fiber cable (which carries the light), and a receiver (which accepts the transmitted light signal and converts it to an electrical signal). A principle called total internal reflection allows optical fibers to retain the light they carry. When light passes from a dense substance into a less dense substance, there is an angle, called the critical angle, beyond which 100 percent of the light is reflected from the surface between substances. Total internal reflection occurs when light strikes the boundary between substances at an angle greater than the critical angle. An optical-fiber core is clad (coated) by a lower density glass layer. Light traveling inside the core of an optical fiber strikes the outside surface at an angle of incidence greater than the critical angle so that all the light is reflected toward the inside of the fiber without loss. As long as the fiber is not curved too sharply, light traveling inside cannot strike the outer surface at less than the critical angle. Thus, light can be transmitted over long distances by being reflected inward thousands of times with no loss.

Use of fiber optics in communications is growing. Fiber-optic communications systems have key advantages over older types of communication. They offer vastly increased bandwidths, allowing tremendous amounts of information to be carried quickly from place to place. They also allow signals to travel for long distances without repeaters, which are needed to compensate for reductions in signal strength. Fiber-optic repeaters are currently about 100 km (about 62 mi) apart, compared to about 1.5 km (about 1 mi) for electrical systems. Many long-distance fiber-optic communications networks for both transcontinental connections and undersea fiber cables for international connections are in operation. Companies such as AT&T, MCI WorldCom, and Sprint have virtually replaced their long-distance copper lines with optical-fiber cables. Local telephone service providers use fiber-optic cables between central office switches and sometimes extend it into neighborhoods and even individual homes. Cable television companies transmit high-bandwidth TV signals to subscribers via fiber-optic cable.